The present invention relates to fluid controllers of the type used to control the flow of fluid from a source of pressurized fluid to a fluid pressure operated device, such as a steering cylinder.
A typical fluid controller of the type to which the present invention relates includes a housing, which defines various fluid ports, and further includes a fluid meter and valving, operable in response to an input, such as the rotation of the vehicle's steering wheel. The typical controller also includes an arrangement for imparting follow up movement to the valving in response to the flow of fluid to the fluid meter. The flow of fluid through the controller valving is proportional to the rate at which the steering wheel is rotated.
Fluid controllers of the type to which the present invention relates are illustrated and described further in U.S. Pat. Nos. Re 25,126, and 3,801,239, both of which are assigned to the assignee of the present invention, and incorporated herein by reference. The controller of U.S. Pat. No. 3,801,239 was developed in response to a problem which has been referred to as "travel limit slip". When the steering cylinder attached to the vehicle steered wheels reaches the end of its stroke, the steered wheels have reached the end of their travel. However, because of leakage out of the fluid meter of the controller, any continuing application of turning force to the steering wheel by the operator will result in the steering wheel continuing to turn, although at a slower rate. The rate of steering wheel rotation at this position (against the "stops") is known as the "travel limit slip rate" (TLSR) and is measured in revolutions per minute of the steering wheel.
U.S. Pat. No. 3,801,239 disclosed the concept of applying a hydraulic "brake" (also referred to as a "plugged star") to brake movement of a moveable member of a fluid meter, relative to the stationary member of the fluid meter. The hydraulic brake solution to the travel limit slip problem has not been put into extensive commercial use, partially because the use of the hydraulic brake in fluid controllers has resulted in a problem known as "feed-through". The conventional fluid controller has utilized a fluid meter comprising a gerotor gear set. A gerotor gear set includes an internally toothed gear (ring) having N+1 internal teeth, and an externally-toothed gear (star) having N external teeth, wherein the star orbits and rotates within the ring. The interengagement of the internal and external teeth defines fluid volume chambers which, in the case of a fluid controller, contain either metered fluid or unmetered fluid. When "feed-through" occurs, there is leakage of metered fluid past the teeth of the star, into a chamber containing unmetered fluid, whenever an excessive pressure differential exists across the star, such as when steering against the stops. Any volume of fluid flowing out of the fluid controller in excess of that actually displaced by the meter, constitutes "feed-through", and is obviously undesirable because it results in movement of the steering cylinder, which does not result from any movement of the steering wheel by the vehicle operator. Such undesirable flow results in a phenomenon known as "wander", i.e. the vehicle "wanders" to either the left or the right, even when the steering wheel is not being rotated.